The present invention relates in general to the pulp and paper industry, and in particular to a new and useful analyzer for monitoring the concentration of lignin in wood pulp and black liquor.
In the pulp and paper industry, the production of paper products requires that lignin be partially removed from the wood chip feed stock prior to making paper products. Lignin is a polymer of complex chemical structure which "cements" the wood's cellulose fibers together. The process by which lignin is removed is referred to as delignification. The most prevalent method of delignification is by chemical means in which raw wood chips and chemicals are combined at a controlled pressure and temperature in a vessel known as a digester. While in the digester, the amount of lignin removed from the wood chips determines the product quality, the product yield, the amount of energy consumed, the quantity of chemicals consumed, and the product cost. Fluid drained from the digester during delignification contains lignin removed from the wood chips and is referred to as "black liquor". The black liquor is used as fuel in a boiler to produce process steam.
The measurement of the residual lignin remaining in the pulp, which exits the digester, is most commonly carried out by laboratory analysis of hourly samples of the digester output (samples are typically obtained at the last stage of the brownstock washer). The lab analysis takes approximately one hour and therefore is a poor method for providing process control feedback and cannot be used for feedforward control. This lab analysis is a back titration method which measures the consumption of the potassium permanganate and is only an approximation to the lignin concentration. The output of titration analysis is referred to as "KAPPA Number" and the procedure is documented in TAPPI procedure T236 hm-88, "KAPPA Number of Pulp". A number of manufacturers have produced automatic sampling and titration devices which have been tried in pulp mill situations but they have been mostly unsuccessful in providing accurate long term results and do not reduce the hour delay between the process and measurement of the residual lignin.
The ultraviolet absorption and fluorescence properties of lignin have long been known and a number of researchers have reported results of measurements in solutions containing lignin. Both the absorption techniques (e.g. see Kleinert, T. N. and Joyce, C. S., "Short Wavelength Ultraviolet Absorption of Various Lignins and Related Substances," Part IV, Pulp and Paper Mag. Can. 58, Oct. 1957, pp. 147-152) and the fluorescence techniques (e.g. see Demas, J. N., Excited State Lifetime Measurements, Academic Press, New York 1983) have all been applied to very dilute solutions. The fluorescence techniques have been used primarily as a method of detecting trace quantities in effluent streams. All of these approaches made use of the very dilute lignin solutions where the absorption and fluorescence signal are linearly related to lignin concentration. The dilution is typically 2,000-10,000 times more dilute than the concentration of lignin in "black liquor" found in the pulping process and thus requires precise sample preparation prior to measurement. A number of devices which attempt to monitor the lignin concentration in "black liquor" during the pulping process by UV absorption techniques (alone or in combination with chemical analysis) have been produced. These devices require sample preparation and dilution prior to measurement and are therefore not in-situ, not real-time, and introduce sampling and dilution errors. See Tikka, P. O., and Virkola, N. E., "A New Kraft Pulping Analyzer for Monitoring Organic and Inorganic Substances", TAPPI Journal, June, 1966, pp. 66-71; Williams, D. J., "The Application of Ultra-Violet Absorption Characteristic of Lignin to the Control of Pulp Uniformity", Appita, Vol. 22, No. 2, September, 1968, pp. 45-52; and Carpart, R., Obese-Jecty, K., Le Cardinal, G. and Gelus, M., "Contribution of the On-Line Kraft Pulping Control", PRP 4 Proceedings, Ghent, 1980.
Use of ultraviolet absorption has recently been extended to the measurement of residual lignin in wood pulp (see Kubulnieks, E., Lundqvist, S., and Pettersson, T., "The STFI OPTI Kappa Analyzer, Applications and Accuracy", TAPPI Journal, November, 1987, pp. 38-42). The device disclosed in this article is marketed by Asea Brown Boveri under the trade name "Opti-Kappa Analyzer". In this approach, the pulp stream is sampled approximately once every 5 minutes. The pulp sample is screened, washed thoroughly, and diluted significantly. The diluted sample is circulated in a loop where UV light absorption is measured over a prescribed time period and the pulp concentration in the slurry (i.e., pulp consistency) is measured independently. This system involves sampling error, screening error, and pulp consistency measurement error. Although the system provides results much faster than the conventional lab titration process, it is still off-line. The washing requirements of this device are stringent since any small amount of black liquor remaining in the diluted solution will absorb UV light and produce error. Bonnier Technology Group (BTG Inc.) also offers a device which operates on a similar principle but uses UV reflection rather than absorption. The BTG device is marketed under the name "KNA-5000 Kappa Number Analyzer".
All of the investigations and devices discussed so far used broad band lamps as the source of UV light. In 1986, researchers at the National Bureau of Standards (see Horvath, J. J., Semerjian, H. G., "Laser Excited Fluorescence Studies of Black Liquor," Proceedings of The SPIE, Vol. 665, June, 1986, pp 258-264) performed fluorescence tests on diluted black liquor samples using a laser as the source of UV light. Although their investigation resulted in better signal-to-noise ratios, they essentially did not extend the art beyond that of previous investigators. They were only able to obtain a functional relationship between fluorescence and lignin concentration in very dilute samples of black liquor (less than 1300 PPM, which is orders of magnitude less than the in-situ concentrations) and did not investigate pulp at all. They did not provide any insight into how one might be able to use either UV absorption or fluorescence techniques to extend the useful measurement range beyond the highly diluted state.
They did mention that this process was a candidate for in-situ monitoring but provided no rational explanation of how the dilution requirement could be overcome. They also mentioned that the measurement could be made more acceptable for field use by using optical fibers to guide the UV excitation light to the process stream and carry the fluorescence signal back to the opto-electronics unit.